1. Field of the Invention
The present invention relates to a charging device for charging an electrostatic latent image support member in an electrophotographic image forming apparatus such as copying machines and printers.
2. Description of Related Art
In an electrophotographic image forming apparatus, a charging device charges a surface of an electrostatic latent image support member such as a photosensitive drum, and the charged area is subjected to image exposure to form an electrostatic latent image. The latent image is visualized by a developing device to form a visible image, which is transferred to transfer material and is fixed thereon.
As is well known in the art, there have been charging devices utilizing corona discharge, and charging devices of a contact type using charging brushes or charging rollers.
Also, U.S. Pat. No. 4,380,384 has disclosed a charging device in which an endless belt, which is driven to rotate, contacts a surface of an electrostatic latent image support member, and a voltage is applied to the belt.
In the charging device, utilizing the corona discharge has such an advantage that the construction is simple and the charging can be stably performed. However, it generates a large amount of ozone, which may deteriorate the electrostatic latent image support member and may adversely affect human bodies.
The charging device using the charging brush or the charging roller has such an advantage that the construction is simple and the amount of generated ozone is extremely small as compared with the corona charging device. However, it is difficult to perform uniform charging, and the reliability is low.
Further, in the charging device using the charging roller, since the roller is pressed against the electrostatic latent image support member with a constant pressing force, the roller tends to deform, which may cause defective charging. This also reduces the reliability.
In the charging device of the belt type disclosed in the U.S. Pat. No. 4,380,384, the amount of the generated ozone is extremely small as compared with the corona charging device, and also has such an advantage over the brush charging and roller charging that an area contacting the electrostatic latent image support member is large so that uniform charging can be performed, resulting in high reliability, and that flexible contact is achieved so that the damage to the electrostatic latent image support member can be suppressed. However, it disadvantageously has a large construction and requires a complicated driving mechanism. These disadvantages cannot be ignored in view of the demand for compact image forming apparatus in these days.